Fail or warn about unsyncable files

Filenames that MacOS sees as ending with .rdump or .idump are reserved
for resource fork and Finder info dumps. They are ignored on the HFS
side and complained about on the local side. This also applies to files
named underscore (_), which are reserved for directory Finder info.

Dotfiles are ignored on both sides (but warned about on the HFS side) to
make some modern tooling invisible to the MacOS.

machfs/directory.py

@@ -2,11 +2,20 @@ from collections.abc import MutableMapping
 import os
 from os import path
 from macresources import make_rez_code, parse_rez_code, make_file, parse_file
+from warnings import warn
 
 
 TEXT_TYPES = [b'TEXT', b'ttro'] # Teach Text read-only
 
 
+def _unsyncability(name): # files named '_' reserved for directory Finder info
+    return name.endswith(('.rdump', '.idump')) or name.startswith('.') or name == '_'
+
+def _fuss_if_unsyncable(name):
+    if _unsyncability(name):
+        raise ValueError('Unsyncable name: %r' % name)
+
+
 class AbstractFolder(MutableMapping):
     def __init__(self, from_dict=()):
         self._prefdict = {} # lowercase to preferred
@@ -103,6 +112,8 @@ class AbstractFolder(MutableMapping):
             filenames[:] = [swapsep(x) for x in filenames if includefilter(x)]
 
             for dn in dirnames:
+                _fuss_if_unsyncable(dn)
+
                 newdir = Folder()
                 newdir.crdate = newdir.mddate = newdir.bkdate = date
                 tmptree[dirpath][dn] = newdir
@@ -110,6 +121,8 @@ class AbstractFolder(MutableMapping):
 
             for fn in filenames:
                 basename = mkbasename(fn)
+                _fuss_if_unsyncable(basename)
+
                 fullbase = path.join(dirpath, basename)
                 fullpath = path.join(dirpath, fn)
 
@@ -173,7 +186,15 @@ class AbstractFolder(MutableMapping):
             return False
 
         written = []
+        blacklist = list()
         for p, obj in self.iter_paths():
+            blacklist_test = ':'.join(p) + ':'
+            if blacklist_test.startswith(tuple(blacklist)): continue
+            if _unsyncability(p[-1]):
+                warn('Ignoring unsyncable name: %r' % (':' + ':'.join(p)))
+                blacklist.append(blacklist_test)
+                continue
+
             nativepath = path.join(folder_path, *(comp.replace(path.sep, ':') for comp in p))
 
             if isinstance(obj, Folder):